SAN FRANCISCO
— Since the dawn of the Nuclear Age, scientists have sought to harness the fundamental process of fusion, which fires the sun and creates the heat necessary for life on Earth.

Its promises are so fantastic as to seem like science fiction: nearly boundless energy, far less waste than current nuclear power, and a recipe that requires readily available elements. So, for decades, the work surrounding this most alluring and elusive quest has been followed with an interest usually reserved only for Mr. Wizard or Martian rocks named Yogi.

Now, researchers claim they may have achieved fusion in miniature, triggering the reaction that stokes the sun's furnace in a Tennessee crucible little bigger than a coffee cup. Instead of jubilation, though, a wary scientific world has responded with skepticism, punctuated only by the slightest expectation of hope.

Given the enormity of the findings and the fact that similar claims have been proven wrong in the past, that's hardly surprising. Yet, if the findings are correct, they surely represent a substantial scientific discovery. They would revolutionize not only how we study fusion reactions but also, possibly, how the future world produces energy.

Even if it is proven false, though, the research still sheds light on the very nature of modern science, where peers armed with spectrometers and spreadsheets spar over the cosmic significance of tiny bubbles and particles smaller than a dust mite. Usually, these tests go on in cloistered labs, far from public notice. But this time, with an issue of such significance - and controversy - they are very much in the open.

"In science, you are guilty until proven innocent," says William Moss of the Lawrence Livermore National Laboratory in California. "This is such an extraordinary claim that you need absolute proof."

The claim is that scientists at Oak Ridge National Laboratory in Tennessee, for an instant, created a star in a jar. Unlike today's nuclear reactors, which take heavy, rare elements such as uranium and split them in a process called fission, the fusion at work in stars "fuses" hydrogen together under immense pressure - releasing far more energy than fission.

Researchers in Tennessee filled a small tube with a specially prepared fluid then shot neutrons through the liquid to generate bubbles. The final ingredient added to the mix were high frequency sound waves. They moved through the fluid, oscillating its pressure between high and low some 20,000 times a second. This, in turn, put pressure on the bubbles.

The bubbles lasted for only a fraction of a second, yet that was long enough for them to grow to nearly one millimeter in diameter - a large size. The energy involved, researchers say, sent temperatures to levels comparable to the sun's core. When the pressure rose again, the bubbles compressed until they imploded, and the violent inrush of liquid, combined with the pressure, resulted in a burst of light called sonoluminescence.

The scientists were hardly blinded by the light, however. Undetectable to the naked human eye, the flash inside the tube had to be registered by light-sensing detectors.

While the scientists who conducted the experiment do not insist that fusion occurred, they suggest that it was likely, given the presence of tritium - an isotope of hydrogen - and neutrons near a particular energy level typical for fusion. Both are byproducts of hydrogen fusion.

"This is our breakthrough," says Richard Lahey Jr. of Rensselaer Polytechnic Institute in Troy, N.Y., an author of the report, which appears this week in the journal Science.

The findings won't send engineers scurrying to build new reactors, though. To take the next step, scientists must find a way to make the process self-sustaining rather than a fleeting phenomenon.

"It's got potential, but it's in the early days," Dr. Lahey says.

Yet others see a different picture. For one, a second group at Oak Ridge tried the experiment, and said it saw nothing. Lahey and colleagues respond that the second group was indeed successful, but their instruments were inefficient.

That isn't enough to satisfy many observers. Dr. Moss says he was twice asked to review the paper and twice rejected it.

The reason: To him, the data were not convincing. The tritium was in such small quantities that it wasn't necessarily the result of fusion, and the researchers didn't show a spike in the thermonuclear neutron spectrum at 2.5 million electron volts - an essential signature of fusion.

"They presented no evidence convincing me that they've actually done what they claimed to have done," he says.

Indeed, throughout the scientific community there is trepidation. Thirteen years ago, scientists at the University of Utah claimed to have induced "cold fusion." The findings were debunked, and the scientific community, many feel, was discredited.

"We've been burned so many times in the past," says Grant Logan of Lawrence Berkeley National Laboratory in California. "Until this is done by others, you have to be very skeptical."

Even Lahey agrees.

Cold fusion "created such a negative feeling that no one is going to believe this until it is reproduced," he says. "When that happens, we'll be able to begin serious discussions about what to do with this."